Hi!
First of all, thank you very much for taking your time and looking into my questions 
Sooo, my issue is next: I have a setter in parent class that worked for the first instance for the subclass (showed “true” when I logged it) but didn’t really work for the second instance (showed undefined). What I did/didn’t do wrong?
class Media {constructor(title){
this._title = title;
this._ratings = [];
this._isCheckedOut = false;
}
get title(){
return this._title;
}
get ratings(){
return this._ratings;
}
get isCheckedOut(){
return this._isChechedOut;
}
set isCheckedOut(value) {
this._isCheckedOut = value;
}
toggleCheckOutStatus(){
this.isCheckedOut = !this.isCheckedOut;
}
getAverageRating(){
let ratingsSum = this.ratings.reduce((currentSum, rating) => currentSum + rating);
return ratingsSum / this.ratings.length;
}
addRating(value) {
this.ratings.push(value)
}};
class Book extends Media {
constructor(author, title, pages) {
super(title);
this._author = author;
this._pages = pages;
}
get author() {
return this._author;
}
get pages() {
return this._pages;
}
};
class Movie extends Media {
constructor(director, title, runTime){
super(title);
this._director = director;
this._runTime = runTime;
}
get director() {
return this._director;
}
get runTime() {
return this._runTime;
}
};
const historyOfEverything = new Book('Bill Bryson', 'A Short History of Nearly Everything', 544);
historyOfEverything.toggleCheckOutStatus();
console.log(historyOfEverything._isCheckedOut);
historyOfEverything.addRating(4);
historyOfEverything.addRating(5);
historyOfEverything.addRating(5);
historyOfEverything.getAverageRating();
console.log(historyOfEverything.getAverageRating());
let speed = new Movie('Jan de Bont','Speed', 116);
speed.toggleCheckOutStatus();
console.log(speed.isCheckedOut);
speed.addRating(1);
speed.addRating(1);
speed.addRating(5);
speed.getAverageRating();
console.log(speed.getAverageRating());